[llvm-dev] New pass manager for optimization pipeline status and questions

[Reid Kleckner via llvm-dev]

Keep in mind that LLVM will continue to support the old pass manager via a
cmake option, so if you are a vendor, you can fallback to the old pass
manager and migrate on your own timeline. However, there are costs to
diverging from upstream. As the community migrates to the NPM, the old pass
manager will become less tested over time, and it may accumulate bugs or
performance regressions.

On Mon, Jan 11, 2021 at 3:45 PM Sjoerd Meijer via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi Alina & Arthur,
>
> I've investigated the performance impact for us and can now say a little
> bit more now where we are. Switching now would lead to performance
> regressions (*) in the initial set of 4 benchmarks we care about. One
> benchmark is overall neutral but shows regressions where we don't really
> want them. Hopefully, your fix for PR48715
> <https://bugs.llvm.org/show_bug.cgi?id=48715> that I raised today will
> solve that, many thanks for the amazingly speedy reply! A second benchmark
> is a bit of disaster, still need to look into that, and a third shows a
> relatively small regression but it's significant for that benchmark and am
> looking into that now, and need to look into the fourth benchmark.
>
> Code-generation is *very* different for the cases I am look at and I am
> profiling and analysing things, for which I need some time. This leads to
> my question: can you remind me about the timelines? I hope we can work in
> tandem to have at least the major issues resolved before we switch.
>
> Thanks for working on this, and for your help and speedy replies,
> Sjoerd.
>
> (*) I am mostly looking at smaller cores, and very tight loops (baremetal)
> and guess that most people look at bigger cores where some of these codegen
> differences have no or a different impact.
>
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